An example of a conventional nitride semiconductor device is a group-III nitride semiconductor field-effect transistor having a heterostructure including a channel layer 1 and a barrier layer 2 as shown in FIG. 15 (Japanese Unexamined Patent Application Publication No. 2013-229493 (PTL 1)). This group-III nitride semiconductor field-effect transistor relates to a field-effect transistor having an insulating film 3 on a surface of a transistor element, and by depositing the insulating film 3 on a surface of the barrier layer 2, the value of a surface level is decreased and the effect of polarization on a hetero interface is increased, so that a nigh output can be attained by increasing a two-dimensional electron gas concentration.
It should be noted reference signs 4, 5, and 6 denote a source electrode, a gate electrode, and a drain electrode, respectively.
Further, another example of a conventional nitride semiconductor device is a field-effect transistor such as that shown in FIG. 16 (Japanese Patent No. 4888115 (PTL 2)). In this field-effect transistor, a drain field plate electrode 7a formed over a drain electrode 7 extends toward a field plate portion 9a of a gate electrode 9 along a surface of a SiON film 10 formed between the drain electrode 7 and the gate electrode 9.
This brings about improvement in withstand voltage by extending the drain field plate length and lessening an electric field at the drain end.